Generally, as fiber materials used in the field where flame resistance is required, for example, there are an asbestos sheet; a ceramic sheet; a glass fiber sheet and a glass fiber thready material; a metal fiber sheet; a metal fiber thready material; and the like. With respect to the asbestos sheet, it has been clearly found that a raw material thereof causes, particularly, a lung cancer, and therefore in the present circumstances, the asbestos sheet has a tendency to be limited in use in various countries. The ceramic sheet is not satisfying pricewise because the cost of the material thereof is so high, although the ceramic sheet has a superior fire-resistant property. The glass fiber sheet and glass fiber thready material have such a problem that borings may be caused due to fusion although they never inflame, and moreover they have not been improved in a fatigue-resistant property against a repeated load. Further, the metal fiber sheet and metal fiber thready material have a disadvantage that they have a very high rigidity and therefore they are not suitable as non-inflammable fiber materials for use in the field where flexibility is required. Therefore, as a practical matter, there is no material which satisfies the requirement for the non-inflammable fiber materials for use in the field where the fatigue-resistant property and the flexibility are required.
Furthermore, as a process for producing thready materials, knitted goods, or woven good, made of rayon fibers which are made non-inflammable, there has been proposed a process in which thready materials, knitted goods, or woven goods, comprising rayon fibers treated with at least one chemical agent selected from a first chemical agent group (a strength increasing agent) consisting of ammonium sulfamate, diammonium imidodisulfonate, ammonium sulfate, ammonium bisulfate, ammonium thiosulfate, ammonium sulfite, and ammonium bisulfite, and at least one chemical agent selected from a second chemical agent group (a flame resistance-improving agent) consisting of ammonium dihydrogen phosphate, diammonium hydrogen phosphate, and guanidine phosphate, are heat treated at from 200.degree. to 260.degree. C. so as to denaturate the rayon fibers into thermally decomposed heat treated carbonized fibers.
The thus treated thready materials, knitted goods, or woven goods are improved in tensile strength because the rayon fibers are prevented from being reduced by burning during the heat treatment step, and moreover they have flame resistance so that they never inflame even if they are made to come into contact with an oxygen-acetylene flame. However, they have such a disadvantage that they are so poor in tear strength that troubles due to insufficient tear strength may occur frequently when they are practically used.